1. Field of the Invention
The present invention relates to a surface acoustic wave apparatus for use as, for example, a resonator or band filter, and more particularly, the present invention relates to an end surface reflection type surface acoustic wave apparatus that utilizes reflection of a surface acoustic wave transmitted between two opposite end surfaces.
2. Description of the Related Art
Surface acoustic wave apparatuses have been widely used as resonators and resonating-type band filters. Now, a surface acoustic wave apparatus that uses a Rayleigh wave as a surface acoustic wave has a structure in which at least one interdigital transducer (hereinafter xe2x80x9cIDTxe2x80x9d) is provided on a piezoelectric substrate, and reflectors are disposed at both sides, in a surface acoustic wave propagation direction, of the region where the IDT is provided. That is, the reflectors are needed to reflect the surface acoustic wave between the reflectors and form a standing wave. As a result, the dimensions of the surface acoustic wave apparatus need to be large.
In contrast to this, there is a known end surface reflection type surface acoustic wave apparatus that uses a Shear Horizontal type (xe2x80x9cSH-typexe2x80x9d) surface acoustic wave. With an SH-type surface acoustic wave such as a BGS wave, etc., the reflectors can be omitted and the two opposite end surfaces of a piezoelectric substrate can be used as reflection end surfaces. Therefore the surface acoustic wave apparatus can be made compact.
Nevertheless, the acoustic velocity of an SH wave in a piezoelectric substrate is slow, so a surface acoustic wave apparatus that uses an SH wave has difficulty handling high frequencies. Also, the bandwidth of a surface acoustic wave filter depends on the electromechanical coupling coefficient k of the piezoelectric substrate, and utilizing an SH-type surface acoustic wave puts limits on the piezoelectric substrate material. Therefore it is not easy to obtain a desired bandwidth due to the restriction of the electromechanical coupling coefficient k.
In order to overcome the problems described above, preferred embodiments of the present invention provide an end surface reflection type surface acoustic wave apparatus that eliminates the defects of prior art, easily handles high frequencies, and easily provides a desired bandwidth.
Prior to the present invention, it was believed that one could not make an end surface reflection type surface acoustic wave apparatus that used a surface acoustic wave other than an SH wave. In contrast to this, the present inventors discovered that an end surface reflection type surface acoustic wave apparatus can be made by using a wave having a longitudinal wave or an Shear Vertical (xe2x80x9cSVxe2x80x9d) wave as the main component of the surface acoustic wave components and by reflecting such a wave between two opposite end surfaces of a surface acoustic wave substrate.
According to a first preferred embodiment of the present invention, an end surface reflection type surface acoustic wave apparatus includes a piezoelectric substrate having a pair of opposite main surfaces and a pair of opposite end surfaces that connect the pair of opposite main surfaces, and at least one interdigital transducer disposed on one main surface of the piezoelectric substrate, wherein a longitudinal wave surface acoustic wave is a main component of surface acoustic waves generated along the piezoelectric substrate.
A Rayleigh wave has two components including a longitudinal wave and an SV wave. These two components are always coupled and propagated. Therefore it was previously considered not possible to use a Rayleigh wave to make a surface acoustic wave apparatus that utilizes end surface reflection.
Nevertheless, with the present invention, a surface acoustic wave having a main component that is a longitudinal wave is reflected at the end surfaces of a substrate without causing mode conversion, as clearly shown in preferred embodiments described later, so it is possible to produce an end surface reflection type surface acoustic wave apparatus that utilizes a longitudinal wave. Moreover, the longitudinal wave has a faster acoustic velocity than an SH wave, so high frequencies can be handled easily. Also, since it is an end surface reflection type surface acoustic wave apparatus, reflectors are not necessary, so the apparatus can be very compact.
According to a preferred embodiment of the present invention, an SH wave component and an SV wave component, other than the longitudinal wave component, each occupy no more than approximately 20% when the energy strength of the entire excited surface acoustic wave is 100%. As a result, essentially only a surface acoustic wave having a longitudinal wave as the main component is reflected.
Also, it is preferred that the longitudinal wave component constitute approximately 70% or more when the energy strength of the entire excited surface acoustic wave is 100%. As a result, only a surface acoustic wave having a longitudinal wave as its main component is reflected, and characteristics deterioration caused by the occurrence of unnecessary modes is prevented. Even more preferred, when the displacements of the excited surface acoustic wave""s longitudinal wave component, an SH component and an SV component are u1, u2, and u3 respectively, is that a ratio between the substrate surface displacements u1 and u2 is about 0.015 or less when the substrate surface is electrically open, and/or about 0.1 or less when the substrate surface is electrically short-circuited, which achieves even better characteristics.
According to another preferred embodiment of the present invention, step differences are provided at intermediate height positions of the pair of opposite end surfaces, and reflection end surfaces are defined by end surface portions that are closer to the main surface side where the IDT is provided than the step differences. Also, the dimensions of the reflection end surfaces in the height direction are in the range of about 5xcex to about 20xcex, where xcex is the wavelength of the surface acoustic wave. By doing so, the surface acoustic wave having a longitudinal wave as a main component is efficiently reflected by the reflection end surfaces. Therefore, for example, grooves having a depth in the range of about 5xcex and about 20xcex are formed in the mother substrate and the substrate is divided at the centers of the grooves to efficiently manufacture a plurality of end surface reflection type surface acoustic wave apparatuses.
The piezoelectric substrate of preferred embodiments of the present invention is not especially limited as long as a surface acoustic wave having a longitudinal wave as its main component can be excited. Examples of this sort of piezoelectric substrate include an Li2B4O7 substrate with Euler angles of approximately (0xc2x0, 40xcx9c60xc2x0, 90xc2x0) or approximately (90xc2x0, 90xc2x0, 20xcx9c50xc2x0), a crystal substrate with Euler angles of approximately (0xc2x0, 120xcx9c140xc2x0, 150xc2x0) or approximately (0xc2x0, 155.25xc2x0, 42xc2x0), an LiNbO3 or LiTaO3 substrate with Euler angles of approximately (90xc2x0, 90xc2x0, 10xcx9c90xc2x0), or other suitable substrates.
According to another preferred embodiment of the present invention, an end surface reflection type surface acoustic wave apparatus includes a piezoelectric substrate having a pair of opposite main surfaces and a pair of opposite end surfaces that connect the pair of opposite main surfaces, and at least one interdigital transducer disposed on one main surface of the piezoelectric substrate, wherein a shear vertical surface acoustic wave is a main component of surface acoustic waves generated along the piezoelectric substrate.
That is, the present inventors discovered that, among excited surface acoustic waves, a surface acoustic wave having an SV wave as its main component is reliably reflected at the opposing end surfaces of a substrate without causing mode conversion. Therefore, the present inventors discovered that it is possible to make an end surface reflection type surface acoustic wave apparatus that uses a surface acoustic wave that has an SV wave as its main component.
An SV wave has a faster acoustic velocity than an SH wave, so an end surface reflection type surface acoustic wave apparatus in accordance with this preferred embodiment of present invention can also handle high frequencies easily. Also, since it is an end surface reflection type surface acoustic wave apparatus, reflectors are not necessary, so the apparatus can be very compact.
According to another preferred embodiment of the present invention, the SH wave component and the longitudinal wave component, other than the SV wave component, each occupy no more than about 20% when the energy strength of the entire excited surface acoustic wave is 100%. As a result, essentially only a surface acoustic wave that has an SV wave as its main component is reflected.
Also, if the SV wave component occupies approximately 70% or more when the energy strength of the entire excited surface acoustic wave is 100%, only a surface acoustic wave that has an SV wave as its main component is efficiently reflected, and characteristics deterioration caused by the occurrence of unnecessary modes is prevented. Even more preferred, when the displacements of the excited surface acoustic wave""s longitudinal wave component, SH component, and SV component are u1, u2, and u3 respectively, is that a ratio u2/u3 is about 0.015 or less when the substrate surface is electrically open, and/or that u2/u3 is about 0.1 or less when the substrate surface is electrically short-circuited, thereby resulting in even better characteristics.
According to another preferred embodiment of the present invention, step differences are provided at intermediate height positions of the pair of opposite end surfaces, and reflection end surfaces are provided of end surface portions that are closer to the main surface where the IDT is provided than the step differences. Also, the dimensions of the reflection end surfaces in the height direction are preferably within the range about 5xcex to about 20xcex, where xcex is the wavelength of the surface acoustic wave. By doing so, the surface acoustic wave having an SV wave as its main component is efficiently reflected by the reflection end surfaces. Therefore, for example, grooves having a depth that is in the range of about 5xcex to about 20xcex are formed in the mother substrate and the substrate is divided at the centers of the grooves to efficiently manufacture a plurality of end surface reflection type surface acoustic wave apparatuses.
The piezoelectric substrate of this preferred embodiment of the present invention is not especially limited as long as a surface acoustic wave having an SV wave as a main component can be excited. An example of this sort of piezoelectric substrate is an Li2B4O7 substrate with Euler angles of approximately (0xc2x0, 45xcx9c80xc2x0, 90xc2x0).
According to another preferred embodiment of the present invention, the preferred embodiments described above are constituted so that the intersection width W of the interdigital transducer with regard to the product of the electromechanical coupling coefficient k and the number N of pairs of interdigital transducers (total number of pairs of longitudinal-coupling-type resonator filters) satisfies the following relationship:
Wxe2x89xa72.1+30.5exe2x88x92(xxe2x88x920.016)/0.055+7.1exe2x88x92(xxe2x88x920.016)/0.012
where x=k2N.
According to another preferred embodiment of the present invention, the intersection width W of the interdigital transducer with regard to the product of the relative permittivity xcex533S* of the piezoelectric substrate and the number N of pairs of interdigital transducers satisfies the following relationship:
Wxe2x89xa7xe2x88x921.68 +25.9exe2x88x92(xxe2x88x9237)/44.8+15.6exe2x88x92(xxe2x88x9237)/216.1
where x=Nxcex533S*.
According to another preferred embodiment of the present invention, an end surface reflection type surface acoustic wave apparatus in accordance with the preferred embodiments described above, is constructed such that an angle formed by the propagation direction of the phase speed of the surface acoustic wave and the end surfaces is approximately 90xc2x15xc2x0, and the angle formed by the upper sections of the end surfaces and the front surface of the substrate is approximately 90xc2x110xc2x0.
According to another preferred embodiment of the present invention, in an end surface reflection type surface acoustic wave apparatus according to preferred embodiments described above, a wavelength of the surface acoustic wave is xcex and there is a single electrode, and the end surface is located at a position that is approximately xcex/2xc2x1xcex/10 toward the outside of the surface acoustic wave propagation direction from the center of the electrode that is inside of the outermost electrode of the interdigital transducer.
In the case of a split electrode defining pairs of electrode fingers with the same charge, the end surface is located at a position that is an integer multiple of approximately xcex/2xc2x1xcex/10 from the center of the electrode fingers forming a pair.
According to another preferred embodiment of the present invention, a communication device includes at least one surface acoustic wave apparatus according to preferred embodiments of the present invention described above.
Other features, elements, characteristics and advantages of the present invention will become apparent from the following detailed description with reference to the attached drawings.